Decoupled temperature and pressure hydrothermal synthesis of carbon sub-micron spheres from cellulose

The temperature and pressure of the hydrothermal process occurring in a batch reactor are typically coupled. Herein, we develop a decoupled temperature and pressure hydrothermal system that can heat the cellulose at a constant pressure, thus lowering the degradation temperature of cellulose significantly and enabling the fast production of carbon sub-micron spheres. Carbon sub-micron spheres can be produced without any isothermal time, much faster compared to the conventional hydrothermal process. High-pressure water can help to cleave the hydrogen bonds in cellulose and facilitate dehydration reactions, thus promoting cellulose carbonization at low temperatures. A life cycle assessment based on a conceptual biorefinery design reveals that this technology leads to a substantial reduction in carbon emissions when hydrochar replacing fuel or used for soil amendment. Overall, the decoupled temperature and pressure hydrothermal treatment in this study provides a promising method to produce sustainable carbon materials from cellulose with a carbon-negative effect.

Constant high pressure promoted the cellulose degradation from 2 to 6 MPa, and lower mass loss was achieved under higher pressures (from 6 to 20 MPa) ( Supplementary   Fig. 3). From 2 to 6 MPa, the main role of pressure is to break the kinetic limits and thus promote the degradation at a low temperature. Above 6 MPa, the release of small molecule products is thermodynamically inhibited, resulting in slightly higher solid yields.
According to proximate analyses, elemental analyses, FTIR, and XRD, high pressures promoted the carbonization of cellulose (Supplementary Table 2  Error bars represent standard deviations of repeated tests. Supplementary  The kinetics of cellulose hydrothermal reaction were calculated with the Coats-Redfern (C-R) method 6 . In general, the hydrothermal reaction rate can be expressed using the first-order rate law 7 : (1 ) where τ is the time of reaction (s); α is the conversion; k is the reaction rate constant (s 1 ) where A is the pre-exponential factor (s -1 ); E is the apparent activation energy (kJ mol -1 ); R is the universal gas constant (kJ mol -1 K -1 ); T is the absolute temperature (K).
In hydrothermal experiments, the heating rate β is constant: dT d    Combining equations above, rearranging and integrating: Rearranging and taking logarithm: The kinetic parameters can be obtained by linear regression of this equation. Proximate analysis revealed that the volatile content decreased from 96.3 wt% (100 C) to 39.5 wt% (300 C), and the fixed carbon increased from 3.7 wt% (100 C) to 60.5 wt% (300 C). The transformation from volatile-rich material into fixed carbon-rich material was consistent with the color change from white to brownish-black (Supplementary Table 3 and Supplementary Fig. 16). Supplementary Figure   The TGA experiments were conducted to evaluate the thermochemical properties of the hydrothermally treated cellulose. In the pyrolysis process (under pure N2), the raw cellulose had one single mass-loss process, which started from 300-315 ℃, with a sharp peak at 353 ℃ and ended at 360380 ℃ ( Supplementary Fig. 24). The pyrolysis of hydrothermal product at 100 ℃ was similar to the untreated cellulose. In contrast, the hydrothermal products at higher temperatures had more stable structures, making them difficult to be thermally decomposed, which might be related to the formation of the aromatic structures reflected in FTIR and Raman spectra. Two peaks at 344358 °C and 433500 C could be detected in the DTG curves of hydrothermally treated cellulose from 150 °C, 200 °C, and 250 °C. However, the first peak at ca. 350 ℃ disappeared in the DTG curve of the hydrochar from 300 °C, indicating the complete decomposition of hydroxy groups and six-member pyran rings in cellulose. Similar to pyrolysis, the DTG curves of cellulose combustion had only one peak, and that of hydrothermal products had two or three peaks, suggesting the transformation from the original cellulose structure to aromatic structures and fixed carbon during the hydrothermal carbonization.
In contrast, no carboxyls or carbonyls were observed from the pyrolysis of hydrochar from 300 °C, indicating the destruction of the inherent structure (the cleavage of hydroxyl and ether bonds) 8,9 . Interestingly, alkenyls could be detected in the FTIR, suggesting the double bonds in the hydrochar. For pyrolysis and combustion experiments in TGA, the kinetics were calculated using the peak analysis-least square method (PA-LSM) 10 . In the parallel reaction kinetic model, the reaction was regarded as the linear combination of a series of independent reactions 11 .
With each peak in the DTG curve representing an independent reaction, the whole reaction was divided into several reactions by peak analysis (PA). The kinetics of each reaction are expressed as 6 : ( In TGA experiments, the heating rate β was constant, rearranging equations: The least-square method (LSM) was used to obtain the Ei, Ai, and ni: where N is the number of data; (dα/dT)exp is the experimental result; (dα/dT)cal is the calculation result. Average deviation index (ADI) was used to evaluate the discrepancy between the experimental and calculation results: is the maximum among the experimental data.
The kinetics of pyrolysis and combustion of the hydrothermally treated cellulose (Supplementary Tables 6 and 7)   The utilization of biomass resources has a great potential in reducing global net carbon emissions when it is used as solid fuel replacing fossil energy or for soil amendment purposes with carbon sequestration benefits. To quantify the sustainability of the DTPH carbonization conceptual biorefinery designs, on a scale-up capacity of 60,000 tonnes per year, a prospective LCA based on process simulation using Aspen Plus®v11 was applied.
This approach has been widely used to quantify the environmental impacts of emerging technology innovations [12][13][14] . Two types of waste biomass, wastepaper sludge (WPS) rich in cellulose and agricultural residue rice straw (RS), were selected as feedstocks in the prospective scenarios. The "cradle-to-grave" system boundary of LCA includes the transportation of WPS or the collection of RS, their DTPH treatment, biogas production in AD and its usage, transportation of products, and their applications in fossil fuel substitution or soil amendment.

Supplementary Figure 26 | Scheme of process designs for WPS and RS DTPH carbonization biorefineries.
(1) Area 100 (A100): DTPH carbonization. Once received at the plant, the biomass feedstock is firstly treated for dedusting and size reduction prior to DTPH carbonization.
Energy consumption is estimated to be 5% of the whole process 15 . Then the biomass feedstock with the reduced size is fed into the reactor, which is filled with water at 20 MPa.
DTPH carbonization reactor is then heated from ambient temperature to 200 °C. Due to the complexity of reactions, a RYIELD-type reactor is chosen 16,17  (2) Area 200 (A200): Anaerobic digestion (AD) and aerobic digestion (AE). Process water from DTPH carbonization is treated by AD and AE before sent to a centralized wastewater treatment (WWT) system. It is suggested that COD removal is expected to be higher than other high solid contenting wastewater stream 16 . In AD, 86% is converted to biogas (methane and carbon dioxide), and 5% is converted to cell mass. Cell mass is produced at a yield of 45 g per kg COD digested 19 . Conversion reaction equations for furfural, HMF, and other polysaccharides degradation products in DTPH carbonization were adopted from NREL process 20 , so as other input materials, such as urea and other additives. Fugitive emissions from the AD were assumed to be 3.00% of the biogas produced 21 , which is then sent to a scrubber for biogas cleaning. The liquid from the Then, 96% of the remaining soluble organic matter is removed, with 74% producing water and carbon dioxide and 22% forming cell mass. The overall COD removal achieves 99.6% after AD and AE. The mass and composition of digestate, as well as electricity consumption of dewatering, were estimated based on NREL processes 19 . The obtained digestate was assumed to be landfilled. (4) Area 400 (A400): Utility. The hot exhaust gas from A300 is sent to the boiler to generate high-pressure steam which is used to heat feedstock and water before flowing to DTPH carbonization and generate electricity for pumps. The steam generated preferentially provides energy to preheat feedstocks and the remaining steam flows to turbine for generating electricity. The exhaust gas is used to dry hydrochar obtained from the press filter before discharging. Cooling water is used to take away the heat generated in reactors. (A400) section.
The described processes were simulated in Aspen Plus ®V11 to generate information for the life cycle inventory. The capacity of simulation is set as 3000 L h -1 , corresponding to a reasonable size of DTPH carbonization reactor operated under high pressure. Process simulation specifications are listed in Supplementary Table 8.
Supplementary  Process water 0.57 g/L a --5.74 g/L b --Note: a Dissolved ash in process water was measured; b Total organic carbon of process water was estimated. c HHV was calculated based on the method in Channiwala and Parikh (2002) 23  For example, RS-SF represents the DTPH carbonization technology process of RS at various B/W ratios with hydrochar used as solid fuel. Since the fossil-based products were substituted, the system expansion allocation method was applied to avoid environmental burdens associated with the conventional products. The 2% cut-off rule was applied, and therefore only major inputs above this threshold are included. Land-use change and infrastructure are excluded from the system.
Life cycle inventory (LCI). Mass and energy flows for cellulose are derived from our inhouse process simulation and corrected with cellulose content for WPS scenarios To describe the carbon positive/negative potential of a technology, carbon positive/negative efficiency is proposed herein, which is defined as the total carbon in the feedstock divided by the carbon that is released or stored. The carbon positive or negative efficiency of different energy conversion technologies was then compared systemically ( Supplementary Fig. 32). While biomass combustion or gasification without CCS are carbon neutral, the negative carbon efficiency of DTPH carbonization technology in this study is higher than that of biomass fermentation, comparable with biomass gasification with CCS, but lower than combustion with CCS. However, the introduction of CCS to biomass gasification or combustion will increase the capital cost and operational cost of the plant significantly, and thus these technologies are not industrially applied currently.
Furthermore, the reaction temperature of DTPH carbonization (~200 C) is lower than that of combustion (750900 C) or gasification (7501150 C).  Fig. 33). DAC, EW, and AR require less land and water; however, EW and AR are limited by the carbon-negative potential, i.e., they cannot meet the 2 °C target with the single system. DAC needs a high energy input (156 EJ yr -1 ), which is 29% of the global energy demand 33 , limiting its investment and development. The DTPH carbonization in this study, together with BECCS, maybe one of the most potential NETs for the 2 °C target, though a significant amount of land and water are required. Therefore, it will be significant to use biomass waste, such as wastepaper sludge, agricultural waste, and forest waste, to save land and water utilization.